In this study the autoradiographic 2-deoxy-D(14C)glucose method of determining glucose utilization was applied in an attempt to demonstrate a circadian rhythm of an intrinsic property of the rat suprachiasmatic nucleus (SCN). It was found in: (1) SCN glucose utilization could be entrained to a 24-hour environmental light-dark cycle and phase shifted 180 degrees by reversing the external entraining cycle. (2) This metabolic rhythm appeared to persist in the absence of any external light-dark cues but it was arrhythmic after 10 days of constant light. (3) The effect of ambient darkness during the light phase of the entraining cycle appeared to "unmask" an underlying endogenous metabolic activity of the nucleus. (4) When animals maintain in constant darkness or constant light were exposed to either ambient light or darkness, respectively, the resulting pattern of altered SCN glucose utilization suggested that the sensitivity of the nucleus to ambient light during constant conditions may have been a function of the phase of an endogenous SCN oscillation.